The invention is generally directed to a compact drying chamber for drying articles by convection heating. More specifically, the invention provides a compact drying chamber for drying printed circuit boards and other electronic assemblies by enhancing evaporation. The compact drying chamber of the invention provides high velocity convection-heated air and maximizes dwell time of printed circuit boards within heated air to achieve enhanced rates of evaporation.
In the field of manufacturing printed circuit boards and electronic assemblies, prior art systems and apparatuses provide various treatments of printed circuit boards such as reflow or wave soldering, etching, and cleaning that are typically followed by rinsing and drying processes. Printed circuit boards are typically cleansed in water-based cleaning systems and then subsequently dried in various types of dryers and drying systems that employ air. Many prior art designs supply air to printed circuit boards at high velocity (typically  greater than 10,000 fpm) to physically remove the bulk of residual water from the surfaces of printed circuit boards and to dry boards by evaporation. Air is typically delivered by a number of discrete air nozzles, air knives or other devices as high velocity heated air streams through which printed circuit boards pass during the drying process.
High velocity air streams striking printed circuit boards blows off the bulk of exposed water residing on the surfaces of boards after cleansing and breaks up such water into small and fine water droplets that are eventually removed by evaporation. Although striking boards with high velocity air readily removes the bulk of residual water on printed circuit boards, the impact breaks water into very small and fine water droplets that are difficult to physically remove from printed circuit boards due to their high surface tension. A layer of air at an intermediate temperature forms between small water droplets and the surrounding air, which produces an insulating or xe2x80x9cskinxe2x80x9d effect that reduces a rate of heat exchange. Impinging small and fine water droplets with high velocity heated air disrupts this xe2x80x9cskinxe2x80x9d effect and enables water to be removed by evaporation.
In addition, small and fine water droplets residing in or around the different types of electronic connectors and components connected or surface-mounted to printed circuit boards are often hidden and essentially removed from the path or flow of heated air. Small and fine water droplets also reside or pool within narrow or deep recesses of electronic connectors and components and, thus, are not sufficiently exposed to the heated airflow. Heated air streams simply cannot reach small and fine water droplets embedded within such electronic connectors and components. Such small and fine water droplets will eventually be removed by evaporation as the temperature of printed circuit boards and electronic connectors and components is raised and maintained upon continued exposure to heated air. However, evaporation takes place at the interface of heated air and water. Since such small and fine water droplets have little or no contact with heated airflow in comparison to their volume, removal of water from electronic connectors and components by evaporation alone would take too long.
At practical speeds of conveyance provided by many prior art dryer designs, the time printed circuit boards are exposed to high velocity heated air, or the dwell time, is insufficient or too short to completely dry printed circuit boards by evaporation. Thus, reliance upon evaporation alone to remove small and fine water droplets from surfaces of printed circuit boards and electronic connectors and components to completely dry boards requires an increase in the dwell time. The more time each point along the surfaces of printed circuit boards is exposed to heated air, the faster the rate of heat exchange and the faster the temperature of printed circuit boards and electronic connectors and components is raised to a sufficient degree to enhance evaporation of water. Hence, the longer the dwell time, the more completely printed circuit boards are dried.
Many prior art dryer designs that employ high velocity heated air increase the dwell time of printed circuit boards by increasing the number of air nozzles or air knives in order to increase the streams or jets of high velocity heated air through which printed circuit boards pass during drying. However, an increase in the number of air nozzles or air knives requires one or more large air blowers and a delivery system to effectively pressurize and circulate a sufficient volume of heated air through a large number of nozzles or knives. Use of large air blowers to accommodate the required output typically results in a substantial increase in the size of the dryer and an increase in the dryer""s power consumption and exhaust requirements. In addition, high power blowers are often limited to intake temperatures of about 125xc2x0 C., which requires cool air to be added to the blowers"" intake, which increases the exhaust requirements of the dryer. For instance, one prior art design employs a large number of discrete air knifes to increase dwell time that requires a twelve foot drying chamber through which printed circuit boards are conveyed to be completely dried. This dryer design is effective in drying printed circuit boards. However, its disadvantages are its large size and high power consumption. In addition, such large dryer designs operate at high noise levels. Another prior art design increases the dwell time of printed circuit boards by delivering a large volume of heated air on the order of about 3,000 cfm at a high velocity that requires three 15 hp air blowers and does not recirculate air. The disadvantages of this dryer design are its very high power consumption and exhaust requirements. Thus, prior art designs that use a large number of air nozzles or air knives and/or a large volume of heated air to lengthen the dwell time of printed circuit boards result in very large dryers and drying systems with high power consumption and high exhaust requirements. Such dryer designs are also expensive to manufacture and operate.
Therefore, it is desirable to provide a dryer or a drying system with the capability of rapidly heating printed circuit boards and particularly electronic connectors and components connected or mounted thereto to efficiently and completely dry printed circuit boards by an enhanced rate of evaporation without substantially increasing the size of the dryer or drying system, its power consumption, exhaust requirements and manufacturing and operating costs.
The invention provides a compact drying chamber for drying printed circuit boards and the different types of electronic connectors and components connected or surface-mounted thereto by convection heating after printed circuit boards have been washed or otherwise processed. Embodiments of the compact drying chamber according to the invention dry printed circuit boards and electronic connectors and components by enhanced evaporation that removes residual water otherwise difficult or impossible to physically remove by heated air streams. Embodiments of the compact drying chamber according to the invention overcome the limitations and disadvantages of prior art dryer designs with respect to overall size, power consumption, exhaust requirements and manufacturing and operating costs.
The invention provides a compact drying chamber comprising features that control the parameters found to significantly affect a rate of evaporation and, consequently, affect a rate at which printed circuit boards are completely dried by evaporation. Such parameters include the velocity at which heated air impacts surfaces of printed circuit boards, the dwell time or the time printed circuit boards are exposed to heated air, and the temperature of the heated air provided to printed circuit boards.
Embodiments of the compact drying chamber of the invention enhance a rate of evaporation of water from printed circuit boards and electronic connectors and components by impacting printed circuit boards with heated air at a high velocity to raise the temperature of boards and connectors and components as quickly as possible to rapidly increase a rate of evaporation and completely dry boards within a relatively short drying time. High velocity heated air also increases the temperature of air within the drying chamber, which also enhances a rate of evaporation.
The compact drying chamber of the invention comprises features that substantially reduce the power consumption of the drying chamber to heat and circulate a sufficient volume of high velocity heated air for effective drying. Specifically, the compact drying chamber comprises an insulated housing that provides a well-insulated drying interior that prevents heat loss during drying operations, thereby requiring less power consumption to heat air. The compact drying chamber also comprises a recirculation system to recirculate the heated air that is enclosed within the insulated housing. The recirculation system includes at least one blower and at least one heater and up to two blowers and four heaters in one embodiment of the invention. Enclosing the air blower within the well-insulated atmosphere of the drying chamber minimizes heat loss and, consequently, keeps power consumption to heat air low. In addition, integrating the air blower within the insulated housing eliminates airflow restrictions that translate into an efficient use of blower power for circulating a sufficient volume of high velocity heated air through the drying chamber.
The drying chamber of the invention comprises overall dimensions that allow a compact design. Integrating the recirculation system and particularly the air blower within the drying chamber keeps the overall size of the drying chamber relatively reduced in comparison with sizes of prior art convection dryers and drying systems. In addition, air jet slots used to produce high velocity air streams or air curtains are integrated into one or more air distribution ducts of the drying chamber, which further provides for a compact design.
The overall dimensions of the compact drying chamber of the invention result in a shortened length of the drying chamber and, hence, a shorter conveyance path through which printed circuit boards are conveyed during drying. Thus, the dwell time of printed circuit boards or exposure of printed circuit boards to high velocity heated air is maximized within practical limits by the compact drying chamber.
The compact drying chamber of the invention also maximizes a velocity pressure at which printed circuit boards are impacted by heated air streams to rapidly increase the temperature of the printed circuit boards and electronic connectors and components and, consequently, to significantly enhance a rate of evaporation therefrom. In particular, the compact drying chamber uses specific air jet configurations that maximize the velocity pressure of air and maximize the efficiency of blower power, thereby keeping power consumption of the drying chamber low.
In addition, the compact design of the drying chamber of the invention allows low cost of manufacturing and low cost parts.